Tool carousel

ABSTRACT

A tool carousel comprising a plurality of wheel sections which can be joined together to form a carousel wheel and a plurality of tool-holding portions. Each tool-holding portion and wheel section is configured such that each tool-holding portion can be journalled to a respective wheel section for rotation between a tool-storage position and a tool-accessing position in which a tool can be installed thereupon or removed therefrom.

This is a division, of application Ser. No. 09/150,967 filed Sep. 11,1998 U.S. Pat. No. 6,155,961.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a tool carousel for use in a tool changer on amachining centre.

2. Background Art

A tool carousel comprises a wheel-like structure which, in use, carriesa series of individually removable tools. In most cases, the carouselcomprises a rotational index means, so as to enable any selected one ofthe tools carried by the carousel wheel to be located at a predeterminedrotational position, whereat a transfer mechanism can present theselected tool to a machine tool for use in a machining operation.

FIG. 1 shows a typical example of a machining centre which includes atool carousel. As can be seen, the tool carousel wheel 1 is mounted upona horizontal rotational axis adjacent a transfer mechanism 2. Referringto FIG. 2, it will be seen that the transfer mechanism 2 comprises atransfer arm 4 for transferring tools between the carousel wheel 1 and amachine tool spindle 3. The transfer arm rotates about a centralrotation axis 8 and has a tool grip at each end.

As can be best seen in FIG. 3, the carousel wheel 1 comprises aplurality of circumferentially consecutive pots 6, each of which is usedfor storing a respective tool. In the storage position, each of the potsorientates its respective tool with its axis generally horizontal.However, when a tool is specified by the machine control, the carouselwheel 1 is rotated until the correct tool is located in the transferposition 5, and the pot containing this tool is then rotated through 90°about a horizontal axis which is perpendicular to the rotational axis ofthe carousel wheel. As a consequence, the pot hangs vertically down andin the manner of the pot designated 7. In this position, the tool hasits axis parallel to the centre line of the machine tool spindle 3. Oncethe pot is in this position, the transfer arm 4 is able to rotate aboutits vertical rotation axis 8 and remove the selected tool from its pot 7whilst simultaneously removing any existing tool from the spindle nose3. As it continues to rotate, the position of the two tools is reversed,the selected tool is presented to the spindle nose, and the deselectedtool is presented to the appropriate pot on the carousel 1. That pot isthen rotated back up through 90°, so that the deselected tool is storedwith its axis inclined horizontally, in common with the other toolsstored in the carousel.

Tools for use in a machine tool are invariably heavy, metal items andthe carousel wheel of a tool carousel therefore has to be strong andsturdy enough to carry all of the tools without buckling or breakingover a long period of time during which the tool carousel is required tooperate reliably, without breakdown. It has therefore been the practiceto form carousel wheels of known tool carousels from metal.

FIG. 4 shows a vertical section through a known tool changerincorporating such a carousel. From the figure, it can be seen that apot 105 has a generally cylindrical form and is attached to the hub 124of a carousel wheel via a rotation axis 117. The pot 105 is fitted witha collar 119 at an end thereof which is situated at the top of the potwhen it is rotated through 90° about the axis 117 at the tool accessposition, as shown in dotted lines in FIG. 4. Axially inwardly of thecollar 119, there is located a retention collar 104 that comprises acentral axial bore into which a number of balls 100 are resilientlyurged to project. In use, a pull-stud of a tool holder is located withinthe bore and gripped by the balls 100. When the tool is to be extracted,this is achieved by axial displacement of an extractor 118 which can beaxially pushed into the bore, to force the pull-stud out.

A dog 102 is provided in the mouth of the pot 105 for retaining thecorrect orientation of the tool holder. A tool identification tag isprovided at 101.

The pot 105 is integrally formed with an arm 103. The arm 103 projectsradially from the side of the pot body and terminates with atransversely extending section, upon which is fitted a rotatable pusherwheel 108 and a bearing 109. The pot is retained in the storage positionshown in FIG. 4 by the action of the bearing 109, which bears against abearing plate 125. The plate is provided with a local slot at the potrelease position, thus enabling the pot to be rotated about the axis117, when it is located there. The pusher wheel 108 is adapted to fitwithin a mouth 123 of a fork 122 mounted at the end of a rod 121 of avertically aligned piston 120. As the carousel wheel rotates, the pusherwheel 108 of each pot assembly consecutively enters the mouth 123 of thefork 122 from the side. When the appropriate tool holder is in position,pneumatic cylinder 120 is actuated, so as to cause the piston rod 120 toextend vertically downwards. As this happens, the pusher wheel 108 isurged downwardly by virtue of the fact that it is constrained within themouth 123 of the fork 122. This downward movement causes the pot 105 torotate about the axis 117, thereby eventually bringing the tool holderinto the position shown in chain-dotted line in FIG. 4. From thisposition, a transfer arm can transfer the tool to the machine head, asgenerally described in relation to FIGS. 1 to 3.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a tool carousel which iseasier and less expensive to manufacture and, if necessary, repair thanthat of the known tool changer.

According to a first aspect of the invention there is provided a toolcarousel according to claim 1. Preferred features of this aspect of theinvention are set out in claims 2 to 21. According to a second aspect ofthe invention, claim 25 provides a lifting and lowering mechanism for atool carousel wheel. A third aspect of the invention is set out in claim28 and provides a drive mechanism for a tool carousel. Preferredfeatures of the various aspects of the invention are set out in thedependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described by way of example andwith reference to the accompanying drawings in which:

FIG. 1 is a schematic front elevation of a machining centreincorporating a tool changer;

FIG. 2 is a schematic plan view of the machining centre shown in FIG. 1;

FIG. 3 is a schematic side elevation of the machining centre shown inFIGS. 1 and 2;

FIG. 4 is a front elevation, partially in cross-section, showing part ofa tool changer in accordance with the prior art;

FIG. 5 is a perspective view of thirty bracket units, assembled togetherto form part of a tool carousel in accordance with the invention;

FIG. 6 is a perspective view of one of the bracket units of FIG. 5 inconjunction with a pot unit and retention collar in accordance with theinvention;

FIGS. 7A, 7B and 8 are perspective views of the bracket unit of FIG. 6;

FIG. 9 is a plan view of the bracket unit of FIGS. 7 and 8;

FIGS. 10A and 10B are perspective views of the pot unit of FIG. 6;

FIG. 11 is a rear elevation of the pot unit of FIGS. 10A and 10B;

FIG. 12 is an underneath view of the pot unit of FIGS. 10 and 11;

FIG. 13 is a perspective view of the retention collar of FIG. 6;

FIG. 14 is a plan view of the retention collar of FIG. 13;

FIG. 15 is a ghost front elevation of a pot lifting and loweringmechanism in accordance with the invention shown in conjunction with theassembly of FIG. 6;

FIG. 16 is a ghost side elevation of the arrangement of FIG. 15;

FIG. 17 is a cross-section of the cam wheel of the pot lifting andlowering mechanism of FIGS. 15 and 16;

FIG. 18 is a front elevation of the cam wheel of FIG. 17;

FIG. 19 is a rear elevation of the cam wheel of FIGS. 17 to 19;

FIG. 20 schematically shows a positioning mechanism for use with acarousel wheel gear index; and

DETAILED DESCRIPTION

FIG. 21 is a graph of rotational speed relative to angular orientationof the shaft of the drive motor shown in FIG. 20.

Referring to FIGS. 5 and 6, it can be seen that the tool carouselaccording to this embodiment of the invention comprises a number ofbracket units 200 which are interlocked to form a carousel wheel 500.Each bracket unit 200 is connected with a respective pot unit 300 andretention collar 400. Each bracket unit 200, pot unit 300 and retentioncollar 400 is injection moulded from a plastics material which comprisesa chemical lubricant. Of course, the components could be manufacturedfrom a different material and an alternative lubricant could be used.

Each of the three primary components shown in FIG. 6 will now bedescribed in detail.

Where the bracket units are described, expressions such as “radial”,“axial” and “circumferential” are used with reference to the assembledcarousel wheel shown in FIG. 5.

One of the bracket units 200 is shown in detail in FIGS. 7A to 9. It canbe seen that the unit comprises a main bracket body portion 202, whichhas a generally wedge-shaped profile when viewed in plan, such as inFIG. 9. On a first radially extending side of the body portion 202,there is provided a male dovetail portion 212. On the opposite radiallyextending side of the body portion 202, there is provided a femaledovetail portion 214. Each bracket unit 200 has the same configuration,so the dovetail formations from two adjacent bracket units 200 can beinterlocked, so as to join them together. Due to the wedge-shape of thebody portion 202, thirty bracket unit can be joined together so as toform a complete ring, thereby defining the carousel wheel shown in FIG.5. Of course, it is not necessary for the carousel ring to be formedfrom thirty bracket assemblies. If it is intended for the carousel wheelto hold a smaller or greater number of tools, then a correspondingnumber of bracket units should be used. In such a case, the bracketunits will need to have the radially extending sides of the body portion202 moulded at an appropriately different angle of separation. Generallyspeaking, if a greater number of bracket assemblies are required inorder to house a respectively larger number of tools, then the anglebetween the two sides of the body portion 202 will be relativelysmaller. On the other hand, if fewer bracket units are required, thenthe angle between the two sides of the body portion 202 would berelatively greater.

This configuration is particularly suitable for manufacture fromplastics materials, hence a lightweight, relatively inexpensive wheelcan be constructed, without sacrificing strength and durability.Furthermore due to its modular construction, the wheel can easily berepaired or modified.

Integrally formed with the body portion 202, there is a hinge arm 204.The hinge arm 204 extends from the radially outer end of the bodyportion 202 and is inclined at an angle of 50° to the carousel wheelaxis. As can be seen particularly clearly in FIGS. 8 and 9, the hingearm 204 is defined by a generally box-like outer wall structure 216which is strengthened by three intersecting cross-webs 218, 220 and 222.An integrally formed cylinder 224 is situated at the intersection of thethree cross-webs 218, 220 and 222. It should be noted that thecross-webs and the cylinder all have walls which are generally parallelwith the wheel axis. This feature can best be seen in FIG. 6.

The box-like section of the hinge arm 204 has generally parallel sides226 and 228. Moving away from the junction of the hinge arm 204 with thebody portion 202, the walls 226 and 228 lead into a distal end portionof a relatively narrower width 230, via inclined walls 232 and 234. Thedistal end portion of the hinge arm 230 is provided with a transverselyextending barrel 236 having a generally circular cross-section. Thebarrel 236 has length which is very slightly longer than the width ofthe distal end portion 236. Each end face 238 of the barrel 236 isprovided with a respective axle lug 240 of a generally circularcross-section.

The barrel 236 comprises a pair of pockets 242, each having arectangular cross-section. The pockets 242 extend in a directionparallel to the cross-webs 218-222 and the wheel axis.

The cross-webs, 218-222, the cylinder 224 and the pockets 242 result ina strong, yet lightweight unit that can be manufactured from arelatively small amount of material.

As can be seen most clearly in FIGS. 7A and 7B, a flexible tongue 206extends perpendicularly from the bracket unit body 202 from a regionclose to the junction of the hinge arm with the body portion 202. Thetongue 206 comprises an elongate hook portion 244 extending transverselyalong its distal end, so as to face generally towards the barrel 236.The tongue 206 is provided with four integrally formed ribs 246 on theopposite side to the hook portion 244 and in the region of the end atwhich it is joined to the body portion 202. The ribs 246 each have athickness which tapers in a curved fashion from the junction of thetongue 206 and the body portion 202 towards the distal end of the tongue206. The ribs 246 serve to constrain the flexion of the tongue 206 in agradually reducing fashion towards its distal end, thereby providing aprecisely controlled spring characteristic. The tongue comprises afurther four ribs 247 on its opposite face, these ribs being directedtowards the barrel and tapering in thickness from the hook portion 244to the axial centre of the tongue. The ribs 247 reduce stresses in thispart of the unit to an acceptable level when the unit is fully loaded.

On a surface of the hinge arm 204 which generally faces the tongue 206,there are provided a pair of stops 248, one of which can be seen clearlyin FIG. 7A. Each stop is located generally towards the side of the hingearm 204 and includes a square-section rebate 250 running in a directiongenerally perpendicular to the axis of the tongue 206.

As can be seen from FIGS. 7A and 7B, the body portion 202 has a generalbox-structure which is strengthened by a pair of further cross-webs 252and 254. This structure also provides strength with low weight andrequires a relatively small amount of material for manufacture.Extending from the tongue-side face of the body portion 202 are a pairof lugs 256 and 258, which extend generally parallel to the tongue 206.Each lug is generally in the form of a cylinder which extends into thebox of the body portion 202.

As can be seen from FIG. 9, the bores of the cylinders 256, 258 extendthrough to the opposite surface of the body portion 202. A furthergenerally cylindrical portion 260 is situated in a crook defined betweenthe cross-webs 252 and 254 and generally towards the opposite end of thebody portion 202. As with cylinders 256 and 258, the bore of cylinder260 extends through to the opposite surface of the body portion 202.

Lugs 256, 258 are used for radial positioning in conjunction with anannular groove in the hub upon which the carousel wheel 500 is mounted.If the annular groove is replaced by a series of accurately bored holes,the lugs can be used for circumferential as well as radial positioningof the bracket units. In such a case, the dovetail formations 212, 214could be omitted.

The opposite surface of the body portion 202 is provided with agenerally trapezoidal wall 208 that extends perpendicularly from theface of the body portion 202 in a direction parallel to the wheel axis.When the bracket units are assembled together to form a carousel wheel,the walls 208 together define a series of radial slots which are used ascarousel-locator slots in a “Geneva wheel” mechanism for controlling therotational position of the carousel wheel.

The end face of the body portion 202 that faces radially inwardly, whenthe bracket units are connected together as shown in FIG. 5, comprises aseries of radially-inwardly facing teeth 210 which, in conjunction withthe teeth provided on the other connected bracket units 200 define acircular gear rack.

In use, the gear rack is used to control the rotational orientation ofthe carousel wheel. Since both the Geneva mechanism and the gear rackhave the same general purpose, one or the other may be deleted. Howeverif both are provided on the bracket units, a choice of rotationalposition control mechanisms is provided, without requiring two differenttypes of bracket unit to be produced.

Referring to FIGS. 6 and 10A to 12, the pot units 300 will now bedescribed.

Each pot unit comprises a tool cylinder 302 integrally formed with ahinge arm 304, which extends generally radially from an outer surface ofthe tool cylinder 302. The hinge arm 304 is hollow and formed from twogenerally planar flank walls 308 joined by a transverse end wall 310 attheir distal ends. Each of the flank walls 308 comprises a circularaperture 312. Due to the natural resilience of the flank walls 308, theapertures 312 snap-fit over the axle lugs 240 provided on a bracketunit. The attachment of a pot unit to a bracket unit in this manner canbe seen clearly in FIG. 6. As an alternative, the hinge arm 204 could beconstructed to provide the necessary resilience to enable the snap-fit.Each flank wall 308 comprises a cut-away portion 314 which has agenerally V-shaped profile, with a somewhat rounded bottom. The cutawayportions 314 are set into the respective edges of the flank walls 308which address a bracket unit when the two are connected together andarranged in the manner of FIG. 6. The cut-away portions 314 serve toaccommodate the box portion 316 of the bracket assembly.

The end wall 310 of the hinge arm 304 is provided with a series ofaxially extending ridges 316 which interlock with ribs 247 provided onthe bracket unit 300.

Each of the ridges 316 terminates in an undercut 322. In use, thetransversely extending hooked portion 244 provided on the tongue 206 ofthe bracket unit 200 snap-fittingly locates underneath the undercut 322when the pot is rotated about the hinge 240, 312 in the clock-wisedirection, to the position shown in FIG. 6. This secures the position ofthe pot unit 300, relative to the bracket unit 200. Although thisclipping method has been found particularly effective, otherarrangements may be employed. For example, a much bigger clip, forgripping a cylinder, may be provided on each bracket unit 200.

On the axially opposite side to the cut-away portions 314, the hinge arm304 is provided with an integrally formed barrel 318. The barrelcomprises a bore 320, which extends in a direction perpendicular to theaxial direction and the radial direction of the cylinder 302. In use,the barrel co-operates with a lifting mechanism which comprises a fork122 for constraining the barrel, the mechanism being used to causerotation of the pot assembly about the hinge 312, 240. Such a mechanismis described below.

The hinge arm 304 further comprises an internal, lateral cross member324 for strength and stiffness. Two further, mutually parallel internalwalls 325 extend perpendicular to the cross member 324. These alsoenhance the stiffness of the structure.

The tool cylinder 302 is provided, at one axial end, with a seat portion306 for accommodating a retention collar 400. Referring to FIG. 10, itwill be seen that the seat portion 306 takes the form of an axiallyextending seat cylinder 326 concentrically situated at one end of thetool cylinder 302. Extending radially into the mouth of the seatcylinder 326, there are provided three lugs 328 at 120° intervals.Circumferentially in line with each lug and axially inwardly of themouth of the cylinder 326, there is provided an elongate recess channel329, which has a part-circular cross-section. A similarly shaped channel330 is provided between each pair of lugs 328 and extends from the mouthof the cylinder 326 to a shoulder 332 which faces axially back towardsthe mouth of the cylinder 326.

Referring to FIGS. 6, 13 and 14, it will be seen that the retentioncollar 400 is generally cylindrical and provided with three radiallyprojecting lugs 402 which are located at 120° intervals about itsperiphery, each at an axial distance which is approximately mid-waybetween the two end surfaces of the holder.

Each lug 402 is in the form of a flexible bridge, which extends across arespective axially extending channel 403. The radially outer surface ofeach lug 402 is provided with an axially extending rib 404, mid-waybetween its two circumferential ends.

The lugs 402 co-operate with the lugs 328 provided in the cylinder 326of the tool cylinder 302. In use, the retention collar 400 is presentedto the cylinder 326, with the ribs 404 circumferentially aligned withthe channels 330 provided on the inner face of the cylinder 326. Theretention collar is then inserted axially into the cylinder 326, untilthe advancing end surface of the collar 400 abuts the shoulder 332. Atthis point, the collar 400 is rotated and the lugs 402 flex radiallyinwardly, as the ribs 404 are urged out of the channels 330. To securethe collar 400 in place, it is rotated until the lugs 402 are eachsituated behind a respective lug 328, at which point the ribs 404 becomecircumferentially aligned with the channels 329 and snap into positiondue to the inherent flexibility of the lugs 402. The combination of thebridge-shape of the lugs 402 and the channels 403 provides sufficientradial flexibility for this operation to be performed. Once the lugs 402are located axially behind the lugs 328, the collar 400 is axiallysecured within the tool cylinder 302. To remove the collar 400 from thetool cylinder 302, the collar 400 must first be rotated against theradial resilience of the lugs 402, until the ribs 404 are once againcircumferentially aligned with the channels 330, whereupon the collarcan be axially withdrawn.

The retention collar 400 further comprises six internal, axiallyextending tongues 406, which are arranged in three groups of two, thegroups being located at 120° intervals. Each tongue 406 is secured atone end to the inner wall of the collar cylinder 400, and, at theopposite end (towards the top of FIG. 13) is unrestrained, therebyenabling each tongue 406 to flex radially. Each tongue is provided witha radially inwardly facing tool-gripping lug 408 proximate to its distalend. In use, the tongues co-operate to grip the pull-stud of a toolholder 600 to secure the tool axially within the cylinder 302. In thisregard, it can be seen in FIG. 6 that the pull-stud of the illustratedtool holder 600 comprises a radial flange 602. In practice, the toolholder is inserted into the tool cylinder 302 from the end of the toolcylinder that is opposite to the end in which the tool holder 400 isinserted. Therefore, the pull-stud of the tool holder enters the collar400 from the bottom of FIG. 13. As the pull-stud moves between the lugs408, the tongues 406 move radially outwardly. Once the flange 602 hasmoved above the lugs 408, as shown in FIG. 13, the tongues snap backinto place, thereby resisting downward movement of the pull-stud 600.

Radially inwardly directed struts 410 serve restrict radial displacementof the pull-stud during insertion, thereby protecting the tongues 406from over-flexion.

As a consequence of the described arrangement, collars having differentinternal dimensions, for holding tool holders configured to differentstandards, may be interchangeably secured within the pot cylinder. It iseven possible to configure a retention collar to be axially reversible;that is to say with means for gripping one type of pull-stud in oneaxial end region and different means for gripping a different type ofpull-stud in the opposite axial end region.

In use, the bracket units are connected together in the manner shown inFIG. 5. Each bracket unit is provided with a respective pot unit 300,these being connected together as shown in FIG. 6. Each pot unit 300 hasa retention collar 400 fitted inside in the manner described above.

The assembled carousel is fitted to a tool changer of the general typeshown in FIG. 1. In this arrangement, the Geneva wheel indexingmechanism will be situated towards the right of the tool carousel, asviewed in FIG. 1, and the open, tool receiving end of each tool cylinder302 will face towards the left of FIG. 1.

The carousel is caused to rotate using either the Geneva wheel mechanismor the gear rack 210, until the desired tool is situated at the bottomof the tool carousel. When in this position, a tool release mechanism(described below) causes the tongue 206 to lift up, thereby allowing thetool cylinder to rotate around the hinge 240, 312. The rotation of thetool cylinder is controlled by a lifting/lowering mechanism (describedbelow) which interacts with the barrel 318. Once the pot unit has beenmoved into a position whereby the tool cylinder has its axis verticallyaligned, the tool can be removed from the cylinder using a tool arm inthe standard manner.

A pot lifting and lowering mechanism 700 will now be described withreference to FIGS. 15 to 19.

The mechanism comprises a vertically-mounted cam wheel 702 which rotateson a horizontal axis 704. The cam wheel comprises a radially outergeared periphery 706 which meshes with drive gearing provided on a drivemotor 708.

The cam wheel 702 comprises two primary camming formations. The first ofthese is a spiral channel 710 which is moulded into a first face of thecam wheel 720. The spiral channel accommodates a lug (not shown) of alifting arm 121, which extends generally vertically, as can be seen inFIG. 16. As the cam wheel 702 is caused to rotate by the motor 708, thelug, which is entrained within the spiral channel and which isconstrained to move only vertically, is caused to move up or down,depending upon the direction of rotation of the cam wheel 702. This, inturn, causes the arm 121 to move up and down and, thus, the fork 122with its mouth 123 moves up and down correspondingly.

On its opposite side, as can best be seen from FIGS. 17 and 20, the camwheel 702 is provided with a generally circumferentially extendingcamming surface 712. This camming surface bears against a release pin714, which is vertically mounted and comprises a lifting catch 716 atits axially lowermost end. The lifting catch 716 is hooked underneath hetongue 206 of the bracket unit currently in position. A helicalcompression spring 718 encircles the release pin 714 between its head720 and the upper surface of a mounting bracket 722, through which itextends. The compression spring 718 urges the release pin 714 upwards,but this action is resisted by the camming surface 712, which bears onthe head of the pin 720.

Referring to FIG. 19, it will be seen that the camming surface 712 has apart-circular portion 713 that extends for 270° about the axis at aconstant, maximum radial distance. Whilst this part-circular bearingsurface 713 bears against the head of the release pin 714, the pin ismaintained in the lowermost position shown in FIG. 16. However, thecamming surface 712 also comprises a chamfered portion 715 defined bytwo flat portions 717, each of which is radially closer to the rotationaxis of the cam wheel 702 than the part-circular portion 713.Consequently, when the cam wheel 702 is rotated to bring the chamferedportion 715 above the release pin 714, the pin is allowed to moveupwardly, under the action of the compression spring 718, and thelifting catch 716 lifts the tongue 206 of the pot unit 200 upwards. Thisreleases the pot unit 300 in such a manner that it can be rotated aboutthe hinge 240.

Due to the relative configurations of the spiral 710 and the cam surface712, at the time that the hinge is lifted upwards, the arm 121 is causedto move downwardly and the fork 122 then begins to push the barrel 318downwardly, thereby causing the pot unit 300 to rotate about the axis.

More specifically, referring to FIG. 19, when the cam wheel 702 isorientated such that position A on its circumference is at the lowermostpoint, the arm and the release pin 714 will be positioned as shown inFIG. 16. If the wheel 702 is then caused to rotate in the clockwisedirection of FIG. 19, the arm 121 is first lifted slightly, so causingthe fork 122 to take the load of the clip 206. The release pin 714 isthen allowed to spring upwards, thereby lifting the tongue 206.Thereafter, the arm 121 is gradually lowered, until point B islowermost, at which time the pot unit has been rotated around the axle240 to such an extent that it will not longer be interfered with by thetongue 206. Therefore, the cam surface 712 once again lowers the catch.As the cam wheel 702 is rotated further in the clockwise direction, thearm 121 is lowered still further until it reaches a lowest point, whencircumferential point C of the cam wheel 702 is lowest. At this point,the tool cylinder 302 has its axis aligned vertically and the tool isready for removal by the transfer arm 4.

To lift the pot unit, the cam wheel 702 is merely rotated in theopposite direction, so as to move the cam wheel 702 anticlockwise asseen in FIG. 19.

FIG. 20 shows a drive mechanism 800 for use in conjunction with thecircular gear track 210. The drive mechanism comprises a motor 801fitted with a drive shaft 802 with a radial gear 804 for meshing withthe drive track 210. The drive shaft is fitted with a steel bar 806 thatrotates as the drive shaft 802 rotates. Three proximity switches 808,810 and 812 are provided along the rotational path of the bar 806. Theseare connected with a control device 814 that controls the rotationalspeed of the motor 801.

The operation of the mechanism will now be described with reference toFIG. 21.

Upon application of a current to the motor, it ramps up to a maximumspeed indicated at W in FIG. 21. The motor continues at this speed untilan end 850 of the bar 806 passes proximity switch 808, as shown inchain-dot line in FIG. 20. Once the proximity switch detects thepresence of the iron bar, the motor is ramped down to an intermediatespeed, the occurrence of which is shown at X in FIG. 21. The motor thencontinues to rotate at the intermediate speed, until proximity switch810 detects the presence of the iron bar. This event is indicated at Yin FIG. 21. It will be seen that the controller 814 then steps the motordown to the minimum rotational speed, until the leading edge of the barend passes proximity switch 812 and the trailing edge of the bar endsimultaneously passes proximity switch 810, when the controller sends asignal for the motor to stop, as indicated at Z in FIG. 21.

This arrangement allows the rotational velocity of the motor to bearrested in a precise and controlled manner that avoids damage to any ofthe components of the carousel.

Whilst the present invention has been described in connection with whatis considered the most practical and preferred embodiment, it is to beunderstood that this invention is not limited to the disclosedembodiment, but is intended to cover various arrangements includingwithin the spirit and scope of the broadest interpretation so as toencompass all such modifications and equivalent arrangements.

We claim:
 1. A drive mechanism for a tool carousel wheel, comprising adrive motor for driving the wheel, a member fitted to the drive shaft ofthe drive motor and rotatable therewith, a plurality of proximityswitches for generating proximity signals as the said member passes andcontrol means for receiving the signals from the proximity switches andcontrolling the rotational speed of the said motor to correspond todifferent speeds with reference to the rotational position of the driveshaft.
 2. A driving mechanism according to claim 1, wherein the controlmeans switches the motor between a relatively fast speed, anintermediate speed and a relatively low speed.
 3. A drive mechanismaccording to claim 2, comprising three proximity switches situated atregular intervals along the path of travel of the said member.